Protein phosphorylation is now well recognized as an important mechanism utilized by cells to transduce signals during different stages of cellular function (Fischer et al, Science 253:401-6 (1991); Flint et al., The EMBO J. 12:1937-46 (1993)). There are at least two major classes of phosphatases: (1) those that dephosphorylate proteins (or peptides) that contain a phosphate group(s) on a serine or threonine moiety (termed Ser/Thr phosphatases) and (2) those that remove a phosphate group(s) from the amino acid tyrosine (termed protein tyrosine phosphatases or PTPases).
The PTPases are a family of enzymes that can be classified into two groups: a) intracellular or nontransmembrane PTPases and b) receptor-type or transmembrane PTPases.
Intracellular PTPases:
Most known intracellular type PTPases contain a single conserved catalytic phosphatase domain consisting of 220-240 amino acid residues. The regions outside the PTPase domains are believed to play important roles in localizing the intracellular PTPases subcellularly (Mauro, L. J. and Dixon, J. E. TIBS 19: 151-155 (1994)). The first intracellular PTPase to be purified and characterized was PTP1B which was isolated from human placenta (Tonks et al., J. Biol. Chem. 263: 6722-6730 (1988)). Shortly after, PTP1B was cloned (Charbonneau et al., Proc. Natl. Acad. Sci. USA 86: 5252-5256 (1989); Chernoff et al., Proc. Natl. Acad. Sci. USA 87: 2735-2789 (1989)). Other examples of intracellular PTPases include (1) T-cell PTPase (Cool et al. Proc. Natl. Acad. Sci. USA 86: 5257-5261 (1989)), (2) rat brain PTPase (Guan et al., Proc. Natl. Acad. Sci. USA 87:1501-1502 (1990)), (3) neuronal phosphatase STEP (Lombroso et al., Proc. Natl. Acad. Sci. USA 88: 7242-7246 (1991)), (4) ezrin-domain containing PTPases: PTPMEG1 (Guet al., Proc. Natl. Acad. Sci. USA 88: 5867-57871 (1991)), PTPH1 (Yang and Tonks, Proc. Natl. Acad. Sci. USA 88: 5949-5953 (1991)), PTPD1 and PTPD2 (Møller et al., Proc. Natl. Acad. Sci. USA 91: 7477-7481 (1994)), FAP-1/BAS (Sato et al., Science 268: 411-415 (1995); Banville et al., J. Biol. Chem. 269: 22320-22327 (1994); Maekawa et al., FEBS Letters 337: 200-206 (1994)), and SH2 domain containing PTPases: PTP1C/SH-PTP1/SHP-1 (Plutzky et al., Proc. Natl. Acad. Sci. USA 89: 1123-1127 (1992); Shen et al., Nature Lond. 352: 736-739 (1991)) and PTP1D/Syp/SH-PTP2/SHP-2 (Vogel et al., Science 259: 1611-1614 (1993); Feng et al., Science 259: 1607-1611 (1993); Bastein et al., Biochem. Biophys. Res. Comm. 196: 124-133 (1993)).
Low molecular weight phosphotyrosine-protein phosphatase (LMW-PTPase) shows very little sequence identity to the intracellular PTPases described above. However, this enzyme belongs to the PTPase family due to the following characteristics: (i) it possesses the PTPase active site motif: Cys-Xxx-Xxx-Xxx-Xxx-Xxx-Arg (Cirri et al., Eur. J. Biochem. 214: 647-657 (1993)); (ii) this Cys residue forms a phospho-intermediate during the catalytic reaction similar to the situation with ‘classical’ PTPases (Cirri et al, supra; Chiarugi et al., FEBS Let. 310: 9-12 (1992)); (iii) the overall folding of the molecule shows a surprising degree of similarity to that of PTP1B and Yersinia PTP (Su et al., Nature 370: 575-578 (1994)).
Receptor-type PTPases consist of a) a putative ligand-binding extracellular domain, b) a transmembrane segment, and c) an intracellular catalytic region. The structures and sizes of the putative ligand-binding extracellular domains of receptor-type PTPases are quite divergent. In contrast, the intracellular catalytic regions of receptor-type PTPases are very homologous to each other and to the intracellular PTPases. Most receptor-type PTPases have two tandemly duplicated catalytic PTPase domains.
The first receptor-type PTPases to be identified were (1) CD45LCA (Ralph, S. J., EMBO J. 6: 1251-1257 (1987)) and (2) LAR (Streuli et al., J. Exp. Med. 168: 1523-1530 (1988)) that were recognized to belong to this class of enzymes based on homology to PTP1B (Charbonneau et al., Proc. Natl. Acad. Sci. USA 86: 5252-5256 (1989)). CD45 is a family of high molecular weight glycoproteins and is one of the most abundant leukocyte cell surface glycoproteins and appears to be exclusively expressed upon cells of the hematopoietic system (Trowbridge and Thomas, Ann. Rev. Immunol. 12: 85-116 (1994)).
The identification of CD45 and LAR as members of the PTPase family was quickly followed by identification and cloning of several different members of the receptor-type PTPase group. Thus, 5 different PTPases, (3) PTPα, (4) PTPβ, (5) PTPδ, (6) PTPε, and (7) PTPζ, were identified in one early study (Krueger et al., EMBO J. 9: 3241-3252 (1990)). Other examples of receptor-type PTPases include (8) PTPγ (Barnea et al., Mol. Cell. Biol. 13: 1497-1506 (1995)) which, like PTPζ (Krueger and Saito, Proc. Nat. Acad. Sci. USA 89: 7417-7421 (1992)) contains a carbonic anhydrase-like domain in the extracellular region, (9) PTPμ (Gebbink et al., FEBS Letters 290: 123-130 (1991)), (10) PTPκ (Jiang et al, Mol. Cell. Biol. 13: 2942-2951 (1993)). Based on structural differences the receptor-type PTPases may be classified into subtypes (Fischer et al., Science 253: 401-406 (1991)): (I) CD45; (II) LAR, PTPd, (11) PTPσ; (III) PTPb, (12) SAP-1 (Matozaki et al., J. Biol. Chem. 269: 2075-2081 (1994)), (13) PTP-U2/GLEPP1 (Seimiya et al., Oncogene 10: 1731-1738 (1995); Thomas et al., J. Biol. Chem. 269: 19953-19962 (1994)), and (14) DEP-1; (IV) PTPa,_PTPe. All receptor-type PTPases except Type IV contain two PTPase domains. Novel PTPases are continuously identified, and it is anticipated that more than 500 different species will be found in the human genome, i.e. close to the predicted size of the protein tyrosine kinase superfamily (Hanks and Hunter, FASEB J. 9: 576-596 (1995)).
PTPases are the biological counterparts to protein tyrosine kinases (PTKs). Therefore, one important function of PTPases is to control, down-regulate, the activity of PTKs. However, a more complex picture of the function of PTPases now emerges. Several studies have shown that some PTPases may actually act as positive mediators of cellular signalling. As an example, the SH2 domain-containing PTP1D seems to act as a positive mediator in insulin-stimulated Ras activation (Noguchi et al., Mol. Cell. Biol. 14: 6674-6682 (1994)) and of growth factor-induced mitogenic signal transduction (Xiao et al., J. Biol. Chem. 269: 21244-21248 (1994)), whereas the homologous PTP1C seems to act as a negative regulator of growth factor-stimulated proliferation (Bignon and Siminovitch, Clin. Immunol. Immunopathol. 73: 168-179 (1994)). Another example of PTPases as positive regulators has been provided by studies designed to define the activation of the Src-family of tyrosine kinases. In particular, several lines of evidence indicate that CD45 is positively regulating the activation of hematopoietic cells, possibly through dephosphorylation of the C-terminal tyrosine of Fyn and Lck (Chan et al., Annu. Rev. Immunol. 12: 555-592 (1994)).
Dual specificity protein tyrosine phosphatases (dsPTPases) define a subclass within the PTPases family that can hydrolyze phosphate from phosphortyrosine as well as from phosphor-serine/threonine. dsPTPases contain the signature. sequence of PTPases: His-Cys-Xxx-Xxx-Gly-Xxx-Xxx-Arg. At least three dsPTPases have been shown to dephosphorylate and inactivate extracellular signal-regulated kinase (ERKs)/mitogen-activated protein kinase (MAPK): MAPK phosphatase (CL100, 3CH 134) (Charles et al., Proc. Natl. Acad. Sci. USA 90: 5292-5296 (1993)); PAC-1 (Ward et al., Nature 367: 651-654 (1994)); rVH6 (Mourey et al., J. Biol. Chem. 271: 3795-3802 (1996)). Transcription of dsPTPases are induced by different stimuli, e.g. oxidative stress or heat shock (Ishibashi et al., J. Biol. Chem. 269: 29897-29902 (1994); Keyse and Emslie, Nature 359: 644-647 (1992)). Further, they may be involved in regulation of the cell cycle: cdc25 (Millar and Russell, Cell 68: 407-410 (1992)); KAP (Hannon et al., Proc. Natl. Acad Sci. USA 91: 1731-1735 (1994)). Interestingly, tyrosine dephosphorylation of cdc2 by a dual specific phosphatase, cdc25, is required for induction of mitosis in yeast (review by Walton and Dixon, Annu. Rev. Biochem. 62: 101-120 (1993)).
PTPases were originally identified and purified from cell and tissue lysates using a variety of artificial substrates and therefore their natural function of dephosphorylation was not well known. Since tyrosine phosphorylation by tyrosine kinases is usually associated with cell proliferation, cell transformation and cell differentiation, it was assumed that PTPases were also associated with these events.
This association has now been proven to be the case with many PTPases. PTP1B, a phosphatase whose structure was recently elucidated (Barford et al., Science 263:1397-1404 (1994)) has been shown to be involved in insulin-induced oocyte maturation (Flint et al., The EMBO J. 12:1937-46 (1993)) and recently it has been suggested that the overexpression of this enzyme may be involved in p185c-erb B2-associated breast and ovarian cancers (Wiener, et al., J. Natl. cancer Inst. 86:372-8 (1994); Weiner et al., Am. J. Obstet. Gynecol. 170:1177-883 (1994)). The insulin-induced oocyte maturation mechanism has been correlated with the ability of PTP1B to block activation of S6 kinase. The association with cancer is recent evidence which suggests that overexpression of PTP1B is statistically correlated with increased levels of p185c-erb B2 in ovarian and breast cancer. The role of PTP1B in the etiology and progression of the disease has not yet been elucidated. Inhibitors of PTP1B may therefore help clarify the role of PTP1B in cancer and in some cases provide therapeutic treatment for certain forms of cancer.
The activity of a number of other newly discussed phosphatases are currently under investigation. Two of these: SHP-1 and Syp/PTP1D/SHPTP2/PTP2C/SHP-2 have recently been implicated in the activation of Platelet Derived Growth Factor and Epidermal Growth Factor induced responses (Li et al., Mole. Cell. Biol. 14:509-17 (1994)). Since both growth factors are involved in normal cell processing as well as disease states such as cancer and arteriosclerosis, it is hypothesized that inhibitors of these phosphatases would also show therapeutic efficacy. Accordingly, the compounds of the present invention which exhibit inhibitory activity against various PTPases, are indicated in the treatment or management of the foregoing diseases.
PTPases: the Insulin Receptor Signalling Pathway/Diabetes
Insulin is an important regulator of different metabolic processes and plays a key role in the control of blood glucose. Defects related to its synthesis or signalling lead to diabetes mellitus. Binding of insulin to its receptor causes rapid (auto)phosphorylation of several tyrosine residues in the intracellular part of the b-subunit. Three closely positioned tyrosine residues (the tyrosine-1150 domain) must all be phosphorylated to obtain full activity of the insulin receptor tyrosine kinase (IRTK) which transmits the signal further downstream by tyrosine phosphorylation of other cellular substrates, including insulin receptor substrate-1 (IRS-1) (Wilden et al., J. Biol. Chem. 267: 16660-16668 (1992); Myers and White, Diabetes 42: 643-650 (1993); Lee and Pilch, Am. J. Physiol. 266: C319-C334 (1994); White et al., J. Biol. Chem. 263: 2969-2980 (1988)). The structural basis for the function of the tyrosine-triplet has been provided by recent X-ray crystallographic studies of IRTK that showed tyrosine-1150 to be autoinhibitory in its unphosphorylated state (Hubbard et al., Nature 372: 746-754 (1994)).
Several studies clearly indicate that the activity of the auto-phosphorylated IRTK can be reversed by dephosphorylation in vitro (reviewed in Goldstein, Receptor 3: 1-15 (1993); Mooney and Anderson, J. Biol. Chem. 264: 6850-6857 (1989)), with the tri-phosphorylated tyrosine-1150 domain being the most sensitive target for protein-tyrosine phosphatases (PTPases) as compared to the di- and mono-phosphorylated forms (King et al., Biochem. J. 275: 413-418 (1991)). It is, therefore, tempting to speculate that this tyrosine-triplet functions as a control switch of IRTK activity. Indeed, the IRTK appears to be tightly regulated by PTP-mediated dephosphorylation in vivo (Khan et al., J. Biol. Chem. 264: 12931-12940 (1989); Faure et al., J. Biol. Chem. 267: 11215-11221 (1992); Rothenberg et al., J. Biol. Chem. 266: 8302-8311 (1991)). The intimate coupling of PTPases to the insulin signalling pathway is further evidenced by the finding that insulin differentially regulates PTPase activity in rat hepatoma cells (Meyerovitch et al, Biochemistry 31: 10338-10344 (1992)) and in livers from alloxan diabetic rats (Boylan et a., J. Clin. Invest 90: 174-179 (1992)).
Relatively little is known about the identity of the PTPases involved in IRTK regulation. However, the existence of PTPases with activity towards the insulin receptor can be demonstrated as indicated above. Further, when the strong PTPase-inhibitor pervanadate is added to whole cells an almost full insulin response can be obtained in adipocytes (Fantus et al., Biochemistry 28: 8864-8871 (1989); Eriksson et al., Diabetologia 39: 235-242 (1995)) and skeletal muscle (Leighton et al., Biochem. J. 276: 289-292 (1991)). In addition, recent studies show that a new class of peroxovanadium compounds act as potent hypoglycemic compounds in vivo (Posner et al., supra). Two of these compounds were demonstrated to be more potent inhibitors of dephosphorylation of the insulin receptor than of the EGF-receptor.
It was recently found that the ubiquitously expressed SH2 domain containing PTPase, PTP1D (Vogel et al., 1993, supra), associates with and dephosphorylates IRS-1, but apparently not the IR itself (Kuhné et al., J. Biol. Chem. 268: 11479-11481 (1993); (Kuhné et al., J. Biol. Chem. 269: 15833-15837 (1994)).
Previous studies suggest that the PTPases responsible for IRTK regulation belong to the class of membrane-associated (Faure et al., J. Biol. Chem. 267: 11215-11221 (1992)) and glycosylated molecules (Häring et al., Biochemistry 23: 3298-3306 (1984); Sale, Adv. Prot. Phosphatases 6: 159-186 (1991)). Hashimoto et al. have proposed that LAR might play a role in the physiological regulation of insulin receptors in intact cells (Hashimoto et al., J. Biol. Chem. 267: 13811-13814 (1992)). Their conclusion was reached by comparing the rate of dephosphorylation/inactivation of purified IR using recombinant PTP1B as well as the cytoplasmic domains of LAR and PTPa. Antisense inhibition was recently used to study the effect of LAR on insulin signalling in a rat hepatoma cell line (Kulas et al., J. Biol. Chem. 270: 2435-2438 (1995)). A suppression of LAR protein levels by about 60 percent was paralleled by an approximately 150 percent increase in insulin-induced auto-phosphorylation. However, only a modest 35 percent increase in IRTK activity was observed, whereas the insulin-dependent phosphatidylinositol 3-kinase (PI 3-kinase) activity was significantly increased by 350 percent. Reduced LAR levels did not alter the basal level of IRTK tyrosine phosphorylation or activity. The authors speculate that LAR could specifically dephosphorylate tyrosine residues that are critical for PI 3-kinase activation either on the insulin receptor itself or on a downstream substrate.
While previous reports indicate a role of PTPa in signal transduction through src activation (Zheng et al., Nature 359: 336-339 (1992); den Hertog et al., EMBO J. 12: 3789-3798 (1993)) and interaction with GRB-2 (den Hertog et al., EMBO J. 13: 3020-3032 (1994); Su et al., J. Biol. Chem. 269: 18731-18734 (1994)), a recent study suggests a function for this phosphatase and its close relative PTPe as negative regulators of the insulin receptor signal (Møller et al., 1995 supra). This study also indicates that receptor-like PTPases play a significant role in regulating the IRTK, whereas intracellular PTPases seem to have little, if any, activity towards the insulin receptor. While it appears that the target of the negative regulatory activity of PTPases a and e is the receptor itself, the downmodulating effect of the intracellular TC-PTP seems to be due to a downstream function in the IR-activated signal. Although PTP1B and TC-PTP are closely related, PTP1B had only little influence on the phosphorylation pattern of insulin-treated cells. Both PTPases have distinct structural features that determine their subcellular localization and thereby their access to defined cellular substrates (Frangione et al., Cell 68: 545-560 (1992); Faure and Posner, Glia 9: 311-314 (1993)). Therefore, the lack of activity of PTP1B and TC-PTP towards the IRTK may, at least in part, be explained by the fact that they do not co-localize with the activated insulin receptor. In support of this view, PTP1B and TC-PTP have been excluded as candidates for the IR-associated PTPases in hepatocytes based on subcellular localization studies (Faure et al., J. Biol. Chem. 267: 11215-11221 (1992)).
The transmembrane PTPase CD45, which is believed to be hematopoietic cell-specific, was in a recent study found to negatively regulate the insulin receptor tyrosine kinase in the human multiple myeloma cell line U266 (Kulas et al., J. Biol. Chem. 271: 755-760 (1996)).
PTPases: Somatostatin
Somatostatin inhibits several biological functions including cellular proliferation (Lamberts et al., Molec. Endocrinol. 8: 1289-1297 (1994)). While part of the antiproliferative activities of somatostatin are secondary to its inhibition of hormone and growth factor secretion (e.g. growth hormone and epidermal growth factor), other antiproliferative effects of somatostatin are due to a direct effect on the target cells. As an example, somatostatin analogs inhibit the growth of pancreatic cancer presumably via stimulation of a single PTPase, or a subset of PTPases, rather than a general activation of PTPase levels in the cells (Liebow et al., Proc. Natl. Acad. Sci. USA 86: 2003-2007 (1989); Colas et al., Eur. J. Biochem. 207: 1017-1024 (1992)). in a recent study it was found that somatostatin stimulation of somatostatin receptors SSTR1, but not SSTR2, stably expressed in CHO-K1 cells can stimulate PTPase activity and that this stimulation is pertussis toxin-sensitive. Whether the inhibitory effect of somatostatin on hormone and growth factor secretion is caused by a similar stimulation of PTPase activity in hormone producing cells remains to be determined.
PTPases: The Immune System/Autoimmunity
Several studies suggest that the receptor-type PTPase CD45 plays a critical role not only for initiation of T cell activation, but also for maintaining the T cell receptor-mediated signalling cascade. These studies are reviewed in: (Weiss A., Ann. Rev. Genet. 25: 487-510 (1991); Chan et al., Annu. Rev. Immunol. 12: 555-592 (1994); Trowbridge and Thomas, Annu. Rev. Immunol. 12: 85-116 (1994)).
CD45 is one of the most abundant of the cell surface glycoproteins and is expressed exclusively on hemopoetic cells. In T cells, it has been shown that CD45 is one of the critical components of the signal transduction machinery of lymphocytes. In particular, evidence has suggested that CD45 phosphatase plays a pivotal role in antigen-stimulated proliferation of T lymphocytes after an antigen has bound to the T cell receptor (Trowbridge, Ann. Rev. Immunol, 12:85-116 (1994)). Several studies suggest that the PTPase activity of CD45 plays a role in the activation of Lck, a lymphocyte-specific member of the Src family protein-tyrosine kinase (Mustelin et al., Proc. Natl. Acad. Sci. USA 86: 6302-6306 (1989); Ostergaard et al., Proc. Natl. Acad. Sci. USA 86: 8959-8963 (1989)). These authors hypothesized that the phosphatase activity of CD45 activates Lck by dephosphorylation of a C-terminal tyrosine residue, which may, in turn, be related to T-cell activation. In a recent study it was found that recombinant p56lck specifically associates with recombinant CD45 cytoplasmic domain protein, but not to the cytoplasmic domain of the related PTPa (Ng et al., J. Biol. Chem. 271: 1295-1300 (1996)). The p56lck-CD45 interaction seems to be mediated via a nonconventional SH2 domain interaction not requiring phosphotyrosine. In immature B cells, another member of the Src family protein-tyrosine kinases, Fyn, seems to be a selective substrate for CD45 compared to Lck and Syk (Katagiri et al., J. Biol. Chem. 270: 27987-27990 (1995)).
Studies using transgenic mice with a mutation for the CD45-exon6 exhibited lacked mature T cells. These mice did not respond to an antigenic challenge with the typical T cell mediated response (Kishihara et al., Cell 74:143-56 (1993)). Inhibitors of CD45 phosphatase would therefore be very effective therapeutic agents in conditions that are associated with autoimmune disease.
CD45 has also been shown to be essential for the antibody mediated degranulation of mast cells (Berger et al., J. Exp. Med. 180:471-6 (1994)). These studies were also done with mice that were CD45-deficient. In this case, an IgE-mediated degranulation was demonstrated in wild type but not CD45-deficient T cells from mice. These data suggest that CD45 inhibitors could also play a role in the symptomatic or therapeutic treatment of allergic disorders.
Another recently discovered PTPase, an inducible lymphoid-specific protein tyrosine phosphatase (HePTP) has also been implicated in the immune response. This phosphatase is expressed in both resting T and B lymphocytes, but not non-hemopoetic cells. Upon stimulation of these cells, mRNA levels from the HePTP gene increase 10-15 fold (Zanke et al., Eur. J. Immunol. 22:235-239 (1992)). In both T and B cells HePTP may function during sustained stimulation to modulate the immune response through dephosphorylation of specific residues. Its exact role, however remains to be defined.
Likewise, the hematopoietic cell specific PTP1C seems to act as a negative regulator and play an essential role in immune cell development. In accordance with the above-mentioned important function of CD45, HePTP and PTP1C, selective PTPase inhibitors may be attractive drug candidates both as immunosuppressors and as immunostimulants. One recent study illustrates the potential of PTPase inhibitors as immunmodulators by demonstrating the capacity of the vanadium-based PTPase inhibitor, BMLOV, to induce apparent B cell selective apoptosis compared to T cells (Schieven et al., J. Biol. Chem. 270: 20824-20831 (1995)).
PTPases: Cell-Cell Interactions/Cancer
Focal adhesion plaques, an in vitro phenomenon in which specific contact points are formed when fibroblasts grow on appropriate substrates, seem to mimic, at least in part, cells and their natural surroundings. Several focal adhesion proteins are phosphorylated on tyrosine residues when fibroblasts adhere to and spread on extracellular matrix (Gumbiner, Neuron 11, 551-564 (1993)). However, aberrant tyrosine phosphorylation of these proteins can lead to cellular transformation. The intimate association between PTPases and focal adhesions is supported by the finding of several intracellular PTPases with ezrin-like N-terminal domains, e.g. PTPMEG1 (Gu et al., Proc. Natl. Acad. Sci. USA 88: 5867-5871 (1991)), PTPH1 (Yang and Tonks, Proc. Natl. Acad. Sci. USA 88: 5949-5953 (1991)) and PTPD1 (Møller et al., Proc. Natl. Acad. Sci. USA 91: 7477-7481 (1994)). The ezrin-like domain show similarity to several proteins that are believed to act as links between the cell membrane and the cytoskeleton. PTPD1 was found to be phosphorylated by and associated with c-src in vitro and is hypothesized to be involved in the regulation of phosphorylation of focal adhesions (Møller et al., supra).
PTPases may oppose the action of tyrosine kinases, including those responsible for phosphorylation of focal adhesion proteins, and may therefore function as natural inhibitors of transformation. TC-PTP, and especially the truncated form of this enzyme (Cool et al., Proc. Natl. Acad. Sci. USA 87: 7280-7284 (1990)), can inhibit the transforming activity of v-erb and v-fms (Lammers et al., J. Biol. Chem. 268: 22456-22462 (1993); Zander et al., Oncogene 8: 1175-1182 (1993)). Moreover, it was found that transformation by the oncogenic form of the HER2/neu gene was suppressed in NIH 3T3 fibroblasts overexpressing PTP1B (Brown-Shimer et al., Cancer Res. 52: 478-482 (1992)).
The expression level of PTP1B was found to be increased in a mammary cell line transformed with neu (Zhay et al., Cancer Res. 53: 2272-2278 (1993)). The intimate relationship between tyrosine kinases and PTPases in the development of cancer is further evidenced by the recent finding that PTPe is highly expressed in murine mammary tumors in transgenic mice over-expressing c-neu and v-Ha-ras, but not c-myc or int-2 (Elson and Leder, J. Biol. Chem. 270: 26116-26122 (1995)). Further, the human gene encoding PTPg was mapped to 3p21, a chromosomal region which is frequently deleted in renal and lung carcinomas (LaForgia et al, Proc. Natl. Acad. Sci. USA 88: 5036-5040 (1991)).
In this context, it seems significant that PTPases appear to be involved in controlling the growth of fibroblasts. In a recent study it was found that Swiss 3T3 cells harvested at high density contain a membrane-associated PTPase whose activity on an average is 8-fold higher than that of cells harvested at low or medium density (Pallen and Tong, Proc. Natl. Acad. Sci. USA 88: 6996-7000 (1991)). It was hypothesized by the authors that density-dependent inhibition of cell growth involves the regulated elevation of the activity of the PTPase(s) in question. In accordance with this view, a novel membrane-bound, receptor-type PTPase, DEP-1, showed enhanced (>=10-fold) expression levels with increasing cell density of WI-38 human embryonic lung fibroblasts and in the AG1518 fibroblast cell line (Östman et al., Proc. Natl. Acad. Sci. USA 91: 9680-9684 (1994)).
Two closely related receptor-type PTPases, PTPκ and PTPμ, can mediate homophilic cell-cell interaction when expressed in non-adherent insect cells, suggesting that these PTPases might have a normal physiological function in cell-to-cell signalling (Gebbink et al., J. Biol. Chem. 268: 16101-16104 (1993); Brady-Kalnay et al., J. Cell Biol. 122: 961-972 (1993); Sap et al., Mol. Cell. Biol. 14: 1-9 (1994)). Interestingly, PTPk and PTPμ do not interact with each other, despite their structural similarity (Zondag et al., J. Biol. Chem. 270: 14247-14250 (1995)). From the studies described above it is apparent that PTPases may play an important role in regulating normal cell growth. However, as pointed out above, recent studies indicate that PTPases may also function as positive mediators of intracellular signalling and thereby induce or enhance mitogenic responses. Increased activity of certain PTPases might therefore result in cellular transformation and tumor formation. Indeed, in one study over-expression of PTPα was found to lead to transformation of rat embryo fibroblasts (Zheng, supra). In addition, a novel PTP, SAP-1, was found to be highly expressed in pancreatic and colorectal cancer cells. SAP-1 is mapped to chromosome 19 region q13.4 and might be related to carcinoembryonic antigen mapped to 19q13.2 (Uchida et al., J. Biol. Chem. 269: 12220-12228 (1994)). Further, the dsPTPase, cdc25, dephosphorylates cdc2 at Thr14/Tyr-1 5 and thereby functions as positive regulator of mitosis (reviewed by Hunter, Cell 80: 225-236 (1995)). Inhibitors of specific PTPases are therefore likely to be of significant therapeutic value in the treatment of certain forms of cancer.
PTPases: Platelet Aggregation
Recent studies indicate that PTPases are centrally involved in platelet aggregation. Agonist-induced platelet activation results in calpain-catalyzed cleavage of PTP1B with a concomitant 2-fold stimulation of PTPase activity (Frangioni et al., EMBO J. 12: 4843-4856 (1993)). The cleavage of PTP1B leads to subcellular relocation of the enzyme and correlates with the transition from reversible to irreversible platelet aggregation in platelet-rich plasma. In addition, the SH2 domain containing PTPase, SHP-1, was found to translocate to the cytoskeleton in platelets after thrombin stimulation in an aggregation-dependent manner (Li et al., FEBS Lett. 343: 89-93 (1994)).
Although some details in the above two studies were recently questioned there is over-all agreement that PTP1B and SHP-1 play significant functional roles in platelet aggregation (Ezumi et al., J. Biol. Chem. 270: 11927-11934 (1995)). In accordance with these observations, treatment of platelets with the PTPase inhibitor pervanadate leads to significant increase in tyrosine phosphorylation, secretion and aggregation (Pumiglia et al., Biochem. J. 286: 441-449 (1992)).
PTPases: Osteoporosis
The rate of bone formation is determined by the number and the activity of osteoblasts, which in term are determined by the rate of proliferation and differentiation of osteoblast progenitor cells, respectively. Histomorphometric studies indicate that the osteoblast number is the primary determinant of the rate of bone formation in humans (Gruber et al., Mineral Electrolyte Metab. 12: 246-254 (1987); reviewed in Lau et al., Biochem. J. 257: 23-36 (1989)). Acid phosphatases/pTPases may be involved in negative regulation of osteoblast proliferation. Thus, fluoride, which has phosphatase inhibitory activity, has been found to increase spinal bone density in osteoporotics by increasing osteoblast proliferation (Lau et al., supra). Consistent with this observation, an osteoblastic acid phosphatase with PTPase activity was found to be highly sensitive to mitogenic concentrations of fluoride (Lau et al., J. Biol. Chem. 260: 4653-4660 (1985); Lau et al., J. Biol. Chem. 262: 1389-1397 (1987); Lau et al., Adv. Protein Phosphatases 4: 165-198 (1987)). Interestingly, it was recently found that the level of membrane-bound PTPase activity was increased dramatically when the osteoblast-like cell line UMR 106.06 was grown on collagen type-I matrix compared to uncoated tissue culture plates. Since a significant increase in PTPase activity was observed in density-dependent growth arrested fibroblasts (Pallen and Tong, Proc. Natl. Acad. Sci. 88: 6996-7000 (1991)), it might be speculated that the increased PTPase activity directly inhibits cell growth. The mitogenic action of fluoride and other phosphatase inhibitors (molybdate and vanadate) may thus be explained by their inhibition of acid phosphatases/PTPases that negatively regulate the cell proliferation of osteoblasts. The complex nature of the involvement of PTPases in bone formation is further suggested by the recent identification of a novel parathyroid regulated, receptor-like PTPase, OST-PTP, expressed in bone and testis (Mauro et al., J. Biol. Chem. 269: 30659-30667 (1994)). OST-PTP is up-regulated following differentiation and matrix formation of primary osteoblasts and subsequently down-regulated in the osteoblasts which are actively mineralizing bone in culture. It may be hypothesized that PTPase inhibitors may prevent differentiation via inhibition of OST-PTP or other PTPases thereby leading to continued proliferation. This would be in agreement with the above-mentioned effects of fluoride and the observation that the tyrosine phosphatase inhibitor orthovanadate appears to enhance osteoblast proliferation and matrix formation (Lau et al., Endocrinology 116: 2463-2468 (1988)). In addition, it was recently observed that vanadate, vanadyl and pervanadate all increased the growth of the osteoblast-like cell line UMR106. Vanadyl and pervanadate were stronger stimulators of cell growth than vanadate. Only vanadate was able to regulate the cell differentiation as measured by cell alkaline phosphatase activity (Cortizo et al., Mol. Cell. Biochem. 145: 97-102 (1995)).
PTPases: Microorganisms
Dixon and coworkers have called attention to the fact that PTPases may be a key element in the pathogenic properties of Yersinia (reviewed in Clemens et al. Molecular Microbiology 5: 2617-2620 (1991)). This finding was rather surprising since tyrosine phosphate is thought to be absent in bacteria. The genus Yersinia comprises 3 species: Y. pestis (responsible for the bubonic plague), Y. pseudoturberculosis and Y. enterocolitica (causing enteritis and mesenteric lymphadenitis). Interestingly, a dual-specificity phosphatase, VH1, has been identified in Vaccinia virus (Guan et al., Nature 350: 359-263 (1991)). These observations indicate that PTPases may play critical roles in microbial and parasitic infections, and they further point to PTPase inhibitors as a novel, putative treatment principle of infectious diseases.